1. Field of the Invention
The invention relates to the field of pre-processing of plastic for injection molding of plastic lenses.
2. Prior Art
Polycarbonate (e.g. Merlon or Lexan) is used for the injection molding of plastic lenses, including prescription lenses. It has generally been accepted that a relatively high percentage of lenses so produced will be rejected as having defects in the form of contaminants (particles) molded into the lenses. By properly cleaning the polycarbonate, a marked decrease in the percentage of defective lenses can result.
The polycarbonate, after extrusion is typically cut into pellets of less than 1/2 inch long by the plastics manufacturer. It is generally purchased in this form and then melted. The pellets have burrs on their ends as a result of the cutting process.
The melting of polycarbonate requires a high temperature. The burrs (because of their smaller surface area) tend to burn as the pellets melt. These burned or blackened burrs show up as defects in the optical lenses. (This problem is not generally recognized in the industry as a cause of defects in lenses.) It should also be noted that in addition to the burrs, various foreign matter melted with the pellets cause defects if the pellets are not properly cleaned.
It is known in the prior art to clean the pellets (with for example, a commercial feed grain cleaner) prior to melting them. However, this cleaning has not been particularly effective. For one reason, unless the burrs are disassociated from the pellets, they cannot be removed by the cleaning process.
As will be seen, the present invention provides a method and apparatus for cleaning the pellets which removes the burrs. Moreover, the invented method and apparatus provides a more thorough removal of other contaminants from the pellets.